The present invention is directed towards the synthesis of high purity deuterated sugars, deuterated nucleobases, deuterated nucleosides and deuterated RNA's of defined sequences which can exhibit biochemically useful and biologically valuable properties, thus having potential for therapeutic uses. The past several decades have seen the development of many RNA and DNA sequences for use in therapeutics, diagnostics, drug design, selective inhibition of an RNA sequence within cellular environments, and blocking a function of different types of RNA present inside the cell. One approach has been the use of antisense technology. Antisense oligonucleotides are useful for specifically inhibiting unwanted gene expression in mammalian cells. Antisense oligonucleotides can be used to hybridize to and inhibit the function of an RNA, typically a messenger RNA, by activating RNase H. Primarily, the oligonucleotides affect the level of the target RNA by activation of RNase H, which cleaves the RNA strand of DNA/RNA hybrids. As a result, antisense oligonucleotides have been proposed for the treatment of diseases. While such technology has the potential to be a powerful tool for all diseases, several issues, including molecule stability, have prevented the technology from being a major disease fighting therapy.
Another approach focuses on silencing gene expression at the mRNA level with nucleic acid-based molecules. RNA interference (RNAi) offers great potential for selective gene inhibition and provides great promise for control and management of various biochemical and pharmacological processes. Early studies illustrated that RNA interference in C. elegans is mediated by 21 and 22 nucleotide RNA sequences, see Fire et al., Nature, 391, 806-811, 1998. This was further confirmed by studies illustrating the general phenomenon of specific inhibition of gene expression by small double stranded RNA's mediated by 21 and 22 nucleotide RNA's, Genes Dev., 15, 188-200, 2001. Simultaneous studies confirmed such phenomenon of specific gene expression by small double stranded (dS) RNAs in invertebrates and vertebrates alike. Various studies have also illustrated the use of RNAi as a powerful tool for selective and specific gene inhibition and regulation, see Nishikura, K., Cell, 107, 415-418, 2001; Nykanen, et al., Cell, 107, 309-321, 2001; Tuschl, T., Nat. Biotechnol., 20, 446-448, 2002; Mittal, V., Nature Rev., 5, 355-365, 2004; Proc. Natl. Acad. Sci. USA, 99, 6047-6052, 2002; Donze, O. & Picard, D., Nucl. Acids. Res., 30, e46, 2002; Sui, G et al., Natl. Acad. Sci. USA, 99, 5515-5520, 2002; Paddison, et al., Genes Dev., 16, 948-959, 2002.
In addition to the use of natural double stranded (ds) RNA sequences, chemically modified RNA have been shown to cause similar or enhanced RNA interference in mammalian cells using 2′-deoxy-2′-fluoro-beta-D-arabinonucleic acid (FANA) into sequences for siRNA activities, see Dowler, et al., Nucl. Acids Res., 34, 1669-1675, 2006. Various other modifications to improve SiRNA properties have been pursued, including alterations in backbone chemistry, 2′-sugar modifications, nucleobase modifications, see reviews Nawrot, B et al., Med. Chem., 6, 913-925, 2006 and Manoharan, M. Curr. Opin. Chem. Biol., 8, 570-579, 2004. While modifications of SiRNA have been tolerated, several studies indicate an increased toxicity and reduced efficacy see Harborth, et al., Antisense Nucleic Acid Drug Dev., 13, 83-105, 2003. Chiu et al. demonstrated that the 2′-O-methyl modification, although maintaining an A form RNA-like helix, does retain SiRNA activity, or in some cases, reduces SiRNA activity depending on the number of such modifications within a sequence, see RNA, 9, 1034-1048, 2003. It has also been shown that extensive 2′-O methyl modification of a sequence can be made in the sense strand without loss of SiRNA activity, see Kraynack, B. A., Baker, B. F., RNA, 12, 163-176, 2006. Bicyclic locked nucleic acids (LNA's) that confer high binding affinity have been introduced in SiRNA sequences, especially when the central region of SiRNA sequence is avoided, see Braash, et al., Biochemistry, 42, 7967-7995, 2003. Similarly, altritol sugar modified oligonucleotides (ANA), which contain rigid conformations, and has been shown to faint degradable duplexes with RNA in a sequence specific manner. In addition, ANAs have been shown to stay in A (RNA type) conformation. Fisher, M., et al., Nucl. Acids Res., 35, 1064-1074, 2007 demonstrated that ANA modified siRNAs targeting MDR1 gene exhibited improved efficacy as compared to unmodified controls, specifically effective when modification was near the 3′-end of sense or anti-sense strand.
Several studies have indicated the potential for siRNA uptake by various delivery systems. Such delivery systems can then be exploited in the development of therapeutics. Cholesterol-conjugated siRNA can achieve delivery into cells and silence gene expression. In addition, lipid conjugated siRNA, bile acids, and long chain fatty acids can mediate siRNA uptake into cells and silence gene expression in vivo. Efficient and selective uptake of siRNA conjugates in tissues is dependent on the maximum association with lipoprotein particles, lipoprotein/receptor interactions and transmembrane protein mediated uptake. High density lipoproteins direct the delivery of siRNA into the liver, gut, kidney and steroidal containing organs. Moreover, LDL directs siRNA primarily to the liver. Studies have indicated that the LDL receptor is involved in the delivery of siRNA. Therefore, it has been proposed that siRNA can be designed with chemical modifications to protect against nuclease degradation, abrogate inflammation, reduce off target gene silencing, and thereby improve effectiveness for target genes. Delivery vehicles or conjugates of lipids and other lipophilic molecules which allow enhanced cellular uptake are essential for therapeutic developments. Such siRNAs are presently being developed for human target validation and interfering with diseases pathways and developing new frontier for drug development.
The 3′-end of sense strand of siRNA can be modified and attachment of ligands is most suited at this end, see for example, Ya-Lin Chiu and Tariq Rana, RNA, 9, 1034-1048, 2003; M. Manoharan, Curr. Opin. Chem. Biol, 6, 570-579, 2004; Nawrot, B. and Sipa, K., Curr. Top. Med. Chem., 6, 913-925, 2006; Scaringe, S., et al. Biotechnol., 22, 326-30, 2004. The introduction of lipophilic or hydrophobic groups and enhancement of siRNA delivery and optimization of targets has been addressed and achieved through bioconjugation. Generally the attachment is performed at the 3′-end of the sense strand, but can be performed on the 3′-end of the anti-sense strand. The design of nuclease resistant siRNA has been the subject of intense research and development in attempts to develop effective therapeutics. Thus base modifications such as 2-thiouridine, pseudouridine, and dihydrouridine have illustrated the effect on conformations of RNA molecules and the associated biological activity, see Sipa et al., RNA, 13, 1301-1316, 2007. Layzer, et al., RNA, 10, 766-771, 2004, illustrated that 2′-modified RNA, especially 2′-fluoro, have great resistance towards nuclease and are biological active in-vivo. Dande et al., Med. Chem., 49, 1624-1634, 2006 used 4′-thio modified sugar nucleosides in combination of 2′-O alkyl modification for improving siRNA properties and RNAi enhancement. Li et al., Biochem. Biophys. Res. Comm., 329, 1026-1030, 2005 and Hall et al., Nucl. Acids Res., 32, 5991-6000, 2004 illustrated the replacement of internucleotide phosphate with phosphorothioate and boranophosphates of siRNAs in vivo.
In addition to in vivo stability and appropriate modification of nucleosides, bioconjugation of siRNA molecules, RNA molecules, aptamers and synthetic DNA molecules require key features for cell membrane permeability. Insufficient cross-membrane cellular uptake limits the utility of siRNAs, other single stranded RNAs, or even various DNA molecules. Thus cholesterol attached at the 3′-end of siRNA has been shown to improve in vivo cell trafficking and therapeutic silencing of the gene, see Soutschek et al., Nature, 432, 173-0178, 2004. In addition to cholesterol, various conjugations have been developed, including natural and synthetic protein transduction domains (PTDs), also called cell permeating peptides (CPPs) or membrane permanent peptides (MPPs). PTDs are short amino acid sequences that are able to interact with the plasma membrane. The uptake of MPP-siRNA conjugates takes place rapidly. Such peptides can be conjugated preferably to the 3′-end of the strand. PEG (polyethylene glycols-oligonucleotide) conjugates have been used in various conjugate complexes and possess significant gene silencing effect after uptake in target cells, see Oishi et al., Am. Chem. Soc., 127, 1624-1625, 2005. Aptamers have been used for site specific delivery of siRNAs. Given that aptamers have high affinity for their targets, conjugates with siRNA act as an excellent delivery system and results in efficient inhibition of the target gene expression, see Chu et al., Nucl. Acids Res., 34(10), e73, 2006. These molecules can be conjugated at the 3′-end of siRNA or other biologically active oligonucleotides. Various lipid conjugations at the 3′-end can be attached to oligonucleotides synthesized by the process described by the invention and can be utilized for efficient internalization of oligonucleotides. The lipophilic moiety consists of a hydroxyl function to synthesize a phosphoramidite. Similarly the lipophilic moiety can have carboxylic function at the terminus. The latter can be coupled to a 3′-amino group having a spacer, synthesized by last addition of amino linkers such as C-6 amino linker amidite, of the reverse synthesized oligonucleotide, to the carboxylic moiety using DCC (dicyclohexyl cabodiimide) or similar coupling reagent, see Paula et al., RNA, 13, 431-456, 2007.
Micro-RNA (miRNA) is a large class of non coding RNAs which have been shown to play a role in gene regulation, see Bartel, D. P. Cell, 116, 281-297, He et al. Nat. Rev. Genet, 5:522-531, 2004; Lagos-Quintana et al., Science, 204:853-858, 2001. It is estimated that there are at least 1000 miRNA scattered across the entire human genome. Many of these miRNAs have been shown to down regulate large numbers of target mRNAs, see Lim et al., Nature, 433:769-773, 2005. Different combinations of miRNAs may be involved in regulation of target gene in mammalian cell. siRNA has been shown to function as miRNAs, see Krek et al., Nat. Genet., 37: 495-500, 2005; Doench et al., Genes Dev., 17:438-442, 2003. Micro-RNAs have great potential as therapeutics and in gene regulation, Hammond, S. M., Trends Mol. Med. 12:99-101, 2006. A vast amount of effort is currently being devoted towards understanding miRNA pathways, their role in development and diseases, and their role in cancer. Additionally, miRNA targets are being developed for therapeutic and diagnostics development. A great number of miRNA are being identified and their role is being determined through microarrays, PCR and informatics. Synthesis of RNA designed to target miRNA also requires RNA synthesis and similar modification, as required for SiRNAs, for stability of RNA and bioconjugation resulting in better cellular uptakes. The instant invention will greatly accelerate the pace of this research and development.
Synthesis of therapeutic grade RNA and siRNA requires modification or labeling of the 3′-end of an oligonucleotide. In the case of siRNA, generally it is the 3′-end of the sense strand. The synthesis of 3′-end modified RNA requiring lipophilic, long chain ligands or chromophores, using 3′ to 5′ synthesis methodology is challenging, and requires corresponding solid support. Such synthesis generally results in low coupling efficiency and lower purity of the final oligonucleotide in general because of a large amount of truncated sequences containing desired hydrophobic modification. The authors of the instant invention approached this problem by developing reverse RNA monomer phosphoramidites for RNA synthesis in the 5′ to 3′-direction. This approach leads to very clean oligonucleotide synthesis, thus allowing for introduction of various modifications at the 3′-end cleanly and efficiently.
In order to increase stability, oligonucleotides containing lipids have been synthesized. Attachment of the lipids provides for efficient delivery of the RNA and an increase in the cellular concentration of the oligonucleotides. Hydrophobic molecules, such as cholesterol, can bind to LDL particles and lipoproteins to activate a delivery process involving these proteins to transport oligonucleotides. Lipped nucleic acids may also reduce the hydrophilicity of oligonucleotides. It has also been shown that lipidoic nucleic acids improve the efficacy of oligonucleotides, see Shea, et al., Proc. Natl. Acad. Sci. USA 86, 6553, 1989; Oberhauser, B., and Wagner, E., Nucleic Acids Res., 20, 533, 1992; Saison-Behmoaras, et al,. The EMBO Journal, 10, 1111, 1991; Reed et al., Bioconjugate Chem., 2, 217, 1991; Polushin, et al., Nucleosides & Nucleotides, 12, 853, 1993; Marasco et al., Tetrahedron Lett., 35, 3029, 1994. A series of hydrophobic groups such as adamantane, eicosenoic acid, cholesterol, and dihexadecyl glycerol were attached to oligodeoxy nucleotide sequences at the 3′-end and were hybridized to complementary RNA sequences. The Tm was found to be unaffected indicating that such groups do not interfere with oligo hybridization properties see Manoharan et al., Tetrahedron Lett., 36, 1995; Manoharan, et al., Tetrahedron Lett., 36, 3651-3654, 1995; Gerlt, J. A. Nucleases, 2nd Edition, Linn, S. M., Lloyd, R. S., Roberts, R. J., Eds. Cold Spring Harbor Laboratory Press, p-10, 1993.
For efficient delivery of synthetic RNA molecules, PEG attachment to various oligonucleotides has shown favorable properties. PEG-oligomers have shown enzymatic stability by preventing fast digestion. The thermal melting behavior was not affected, thereby retaining properties of double strand formation. Srivastava et al., Nucleic Acids Symposium Series, 2008, 52, 103-104 recently developed a reverse RNA synthesis process for clean attachment of lipophilic and large molecules to synthetic RNA.